1. Field of the Invention
The present invention relates to carbon electrodes coated with a porous metal film and to its fabrication method and a lithium ion secondary battery using the same.
2. Description of the Background Art
A conventional lithium-ion secondary battery uses a compound which includes lithium, such as LiCoO2 or LiMn2O4, as a cathode. Thus, such a battery is fabricated without having lithium in a carbon electrode used as an anode.
In case of the use of a carbon electrode in a lithium ion secondary battery, in the initial charging of the battery, a passive film is formed on the surface of the carbon electrode. The passive film improves a stabilization of the carbon structure and the reversibility of the carbon electrode, making it possible to use the carbon electrode as an anode of lithium-ion secondary battery.
However, such formation of a passive film is an irreversible reaction, bringing about a contrary effect that it causes the consumption of lithium ions and thereby a reduction in the capacity of the battery, as well as degradation of the life cycle of the battery.
In order to compensate for this loss of lithium ions which occurs during the initial charging of the battery, it is known to provide the battery with an excess amount of lithium ions, by a so-called xe2x80x9ccathode loadingxe2x80x9d in which the carbon electrode is pre-lithiated by intercalating lithium therein.
Conventionally, there are two known methods for prelithiating the carbon electrode; one is a method for prelithiating a carbon active material by a physicochemical process and fabricating an electrode (K. Zaguib, R. Yazami and M. Broussely, 8th International Meeting on Lithium Batteries, 192(1996)), and the other is a method for prelithiating a carbon electrode electrochemically (X. Y. Song and K. Kinoshita, J. Electrochem. Soc., 143, L120(1996), I. I. Olsen and R. Koksbang, U.S. Pat. No. 5,595,837 (1997)).
Since the physicochemical prelithiation process needs to be performed at a high temperature, there is a risk of a fire or an explosion.
Comparatively, the electrochemical prelithiation process can be performed at room temperature, but there is a difficulty in its processing. Especially, this method is disadvantageous in that, since the lithium reaction rate is adjusted by adjusting a current by using a power supply, a power source must be installed and controlled. In addition, since there is no stabilizing process, lithium is not evenly dispersed in the carbon electrode, resulting in a problem that the battery characteristics, especially, a life cycle characteristic, are deteriorated.
Meanwhile, as for the carbon electrode, since the carbon lattice constant is much changed during charging and discharging, the carbon active material is gradually released from an electronic conduction path with repeated charging and discharging, causing problems of reduction of electrode capacity, degradation of a high rate charging and discharging characteristic or degradation of the life cycle.
In an effort to offset the reduction in the conductivity of the carbon active material, generally, carbon having a favorable conductivity such as acetylene black (AB) is added as a conductive material to fabricate an electrode, which is, however, yet to be much improved in the aspect of battery performance.
As an improvement, there have been proposed a method in which a metal having a good conductivity such as silver is added after being reduced (Korea Technochemistry Association, General Meeting, Spring, Excerpt of a Thesis, pp.154xcx9c156, May 9xcx9c10, 1998), or a method in which, in order to improve the electrode capacity, tin oxide or the like is precipitated in carbon and more than two types of cathode active materials are used together (38th Open Discussion for Battery, pp.207xcx9c208, Osaka, Japan, November 11xcx9c13, 1997).
However, those methods are performed in an aqueous solution, which requires washing and drying processes. In addition, a costly reagent must be used, its processes are difficult, a high fabrication cost is incurred, and the adherence of the coated film is not good.
Therefore, an object of the present invention is to provide carbon electrode coated with a porous metal film and its fabrication method and a lithium-ion secondary battery using the same, that are capable of preventing capacity degradation of the carbon electrode caused due to an irreversible reaction generated at the initial state of charging, capable of improving a life cycle by compensating for the amount of lithium consumed due to a low efficiency in charging and discharging, and capable of simplifying the fabrication process of a secondary battery to reduce its fabrication cost.
To achieve at least the above objects in whole or in parts, there is provided carbon electrode for a lithium-ion secondary battery, coated with a porous metal film having a thickness of a few xc3x85xcx9ca few xcexcm.
To achieve at least these advantages in whole or in parts, there is further provided a method for fabricating carbon electrode coated with a porous metal film including the steps of: positioning a carbon electrode roll within a vacuum chamber; winding the carbon electrodes off the roll at a certain speed, winding the carbon electrodes on a different roll and coating a porous metal with a thickness of a few xc3x85xcx9ca few xcexcm on the carbon electrode between the two rolls from a metal evaporation source; and stabilizing the carbon electrode in a vacuum state for a predetermined time period at a predetermined temperature.
To achieve at least these advantages in whole or in parts, there is further provided a lithium-ion secondary battery having carbon electrode coated with a porous metal thin film with a thickness of a few xc3x85xcx9ca few xcexcm and an cathode comprising LiCoO2, LiMn2O4, LiNiO2, V6O13 or V2O5.
Additional advantages, objects, and features of the invention will be set forth in part in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from practice of the invention. The objects and advantages of the invention may be realized and attained as particularly pointed out in the appended claims.